Stable isotope insights into artificial reef effects of floating offshore energy structures in Norwegian North Sea codfishes

Offshore energy structures introduce hard substrate to soft substrate-dominant habitats and may act as artificial reefs providing shelter and food to aggregating fish. In the Northeast Atlantic, knowledge on these effects is limited to shallow habitats in the southern North Sea. Given that effects may be misinterpreted as ‘nature positive’ contributions, or underestimated and impacting ecosystem services like fisheries, this data-gap hinders management. This is especially problematic for the rapid developments of floating offshore wind farms (OWFs), and decommissioning of floating oil and gas (O&G) platforms in deep (>100 m) habitats of Norway. In this study, we analysed the stable isotopic composition of muscle and liver and the condition of three codfishes of commercial importance (saithe; Pollachius virens , tusk; Brosme brosme and ling; Molva molva ) at a floating OWF and two floating O&G platforms off Norway to evaluate how codfish diet and habitat use may be altered by the structures. We find that differences in carbon, nitrogen and sulphur stable isotopes between offshore energy sites and control sites were lower for liver measurements (weeks prior to capture) than muscle (months prior to capture), indicating that codfish diet and habitat use was less impacted by offshore energy structures than longer-term natural feeding variation. Size explained some isotopic differences between sites in saithe, and condition differences between sites in tusk; suggesting that the diet and habitat use of the three species is not significantly impacted by offshore structures. However, we found evidence of lower condition in ling at Hywind Tampen OWF, corresponding to lower nitrogen isotope liver values in ling; that may indicate trade-offs in shelter and diet provision. Overall we highlight the need for further research on trophic effects of deep offshore energy structures to evaluate implications for management and conservation.